1. Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to a color cathode ray tube system having an improved unitized, three-beam, in-line electron gun. The system and associated electron gun according to the invention have application to all types of color television picture tubes, including those used in home entertainment television receivers. The system is especially valuable when applied to special-purpose high-resolution color image tubes that require beam spots that are exceptionally small in diameter, uniorm in size, and symmetrical all over the screen. Tubes of this type include medium-resolution and high-resolution monitors. An example of such a special-purpose tube is one that has a flat faceplate and an associated foil tension mask; a tube of this type is described and claimed in referent copending application Ser. No. 832,493 of common ownership herewith.
Desired performance characte-ristics of color cathode ray tube systems include high resolution, picture brightness, and color purity. Resolution is largely a function of the size and symmetry of the beam spots projected by the electron gun of the tube. Beam spots are desirably small, round, and uniform in size at all points of landing on the screen. Achievement of these ideals is difficult because of the many factors which exert an influence on beam spot configuration. As a result of such factors, beam spots that are smal- and symmetrical at the center of the picture imaging field can become distorted at the periphery of the field, for reason which will be described.
Key factors which influence beam spot size, uniformity and symmetry in picture tubes include the following:
(a) electron gun design, especially the design of the means for focusing and converging the beams in three-beam, in-line guns;
(b) potential of the cathode ray tube screen;
(c) magnitude of the beam current;
(d) the "throw" distance from the electron gun to the screen; and,
(e) the magnitude of beam-distorting influences, such as astigmatism engendered by a self-converging yoke, or that inherent in the gun design.
The ability of an electron gun to form small, symmetrical beam spots is a major factor in achieving optimum resolution. The task of designing guns with this capability has become more challenging because of the reduction in diameter of the CRT neck. This physical constraint has been largely overcome by new, more effective gun designs, such as the gun having an extended field main focus lens described and claimed in U.S. Pat. No. 3,995,194 assigned to the assignee of this invention.
Convergence of the three beams of an in-line electron gun is provided in present-day television systems primarily by the self-converging yoke. This type of yoke is a hybrid having toroidal-type vertical deflection coils and saddle-type horizontal deflection coils. The yoke contains windings which produce an astigmatic field component that has the effect of maintaining the beams in convergence as they are swept across the screen. The converging effect is shown highly schematically in FIG. 1, in which an electron gun 10 is depicted graphically as emitting three beams 12, 13 and 14 which diverge from a common plane 16 to impinge on a curved screen 18. The three beams are shown as being converged at the center point 20 of the screen 18. Due to the effect of the self-convrging yoke, the three beams are also caused to be in convergence at the side of the screen 18, as indicated by point 22, even though the distance that beams must travl from the plane of deflection 16 t point 22 is greater than from the plane of deflection 16 to center point 20 of the screen.
The convergence achieved is not without cost, however, as the beam spots are subject to distortion in the peripheral areas of the screen. The distortion is acceptable in conventional tubes that have the curved screen as the benefits and costs savings of the self-converging yoke outweigh its liabilities.
However, when the screen is flat, as indicated by screen 24 in FIG. 2, the self-converging yoke is unable to maintain beam convergence, as indicated by the spread of the beam spots 28 at the sides 26 of screen 24. In addition to the spread, the spots 28 will be noted as being elongated. This elongation is due primarily to the self-converging yoke. The astigmatic field component, while self-converging the beams, undesirably induces deflection defocusing of the beams when the beams are deflected away from the screen center. The effect is indicated diagrammatically in FIG. 3 by beam spots 34. The elongation of the beam spots at the peripheries of the faceplate, and the relative increase in spot size, is indicated in greater detail in FIG. 3A. The beam spots 34 will be seen as comprising a bright core 34A, and transverse to the core, a dim "halo," 34B. The size and contour of the center beam spot 36 is indicated to illustrate the magnitude of the spot size increase and distortion at the corners of the screen. Attempts to focus such beams are largely ineffectual due to the astigmatic effect--focusing merely results in what appears to be a "rotation" of the spot in that the core becomes the halo and the halo becomes the core, without restoration of center-screen dot contour and size.
The distortion of the beam spots at screen peripheries is attributable to the nature of the field of the yoke that provides the desired beam convergence. The field produced by the yoke has the shape of a pin-cushion for the horizontal deflection component, and the shape of a barrel for the vertical deflection component. In addition to the dipole effect which deflects the beams, a quadrupole, astigmatizing effect is also produced which distorts the beams at the screen peripheries, as indicated in FIGS. 3 and 3A.
As has been noted, the effect is tolerable in conventional tubes where the screen is curved, as shown by FIG. 1, and it is acceptably within the capability of the self-converging yoke to converge the beams without undue distortion. However, when the screen is flat, as indicated by FIG. 2, the astigmatic effect of the self-converging yoke is less tolerable, especially in high-resolution cathode ray tubes. Attempts to further modify the configuration of the self-converging yoke field to adapt it to a flat screen may well increase distortion outside the limits of acceptability. The self-converging abiity of the yoke was already stretched to its limit in its application to the curved screen, before the advent of the flat tension mask tube.
2. Prior Art
Prior art structures for statically converging electron beams have relied upon a variety of techniques such as the use of maghetic influences within and/or outside the tube envelope, and the use of electrostatically charged plates. Also, the prior art shows many examples of causing static beam convergence by inducing an asymmety in an electrostatic field formed at the interface of two spaced electrodes. Prior art techniques for inducing electrostatic field asymmetry have included offsetting the apertures in the opposing faces of two electrodes, and slanting one or more of the opposing faces so that the space lying between is in the form of a wedge-techniques described in U.S. Pat. No. 4,058,753 of common ownership herewith, and in U.S. Pat. No. 2,957,106.
Dynamic convergence means is described in U.S. Pat. No. 3,448,316. Three in-line electron beams generated by three cathodes cross over in the electrostatic field of a main lens. The center beam (green) follows a straight-line path, but the two outer "red" and "blue" beams exit the lens in divergent paths. The beams paths are refracted to converge by electron prisms that enclose the beams, and which are located beyond the exit-point of the beams from the gun. The potential on the outer ones of the electron prisms is made adjustable to provide for static convergence of the red and blue beams at the shadow mask. The center beam is unaffected as the potential on the two inner plates through which it passes is left unchanged. Dynamic convergence is attained by changing the convergence control voltage on the outer prisms at the horizontal scanning frequency. The waveform of the convergence voltage is in the form of a parabola.
In U.S. Pat. No. 4,520,292, von Hekken et al discloses means formed in the screen grid of an electron gun for urging the outer two beams of a three-beam electron gun into convergence with the center beam. The screen grid configuration includes a transversely disposed recessed portion having a substantially rectangular central portion and substantially triangular end parts. The total effect is to tilt the field lines within the reessed portion so that the outer beams converge.
In U.S. Pat. No. 4,058,753, of common ownership herewith, there is disclosed a three-beam electron gun for a color cathode ray tube having an extended field main focus lens means. The focus lens means has for each beam at least three electrodes including a focus electrode for receiving a variable potential for electrically adjusting the focus of the beam. In succession down-beam, there are at least two associated electrodes having potentials thereon which form in the gaps between adjacent electrodes significant main focus field components. To adjust beam focus, the strength of a first of these components is controlled by adjustment of the voltage received by the focus electrode. The strength of the second of the field components is relatively less than that of the first component. Each of the lens means is characterized by having addressing faces of the associated electrodes which define the second field component being so structured and disposed as to cause the second field component-to be asymmetrical and effective to significantly divert the beam from its path in convergence of the beams without any significant distortion of the beam, and substantially independently of any beam-focusing adjustments of the first field component. Electrode structures for producing asymmetric field components include a gap angled forwardly and outwardly, a wedge-shaped gap, and radially offset apertures.
An electron gun system providing beam convergence for use in a color CRT display system is disclosed in referent copending application Ser. No. 921,168. Means including cathode means develop three electron beams, two of which are off-axis with respect to a center axis of the gun. A plurality of electrodes means provide shaping and focusing and assist in the converging of the beams at the screen. Means are provided for developing and applying to the electrode means a pattern of potentials which form field components in the gaps therebetween; at least one of the electrode means receives a varying dynamic focusing voltage for dynamically focusing the beams as they are deflected across the screen. At least selected ones of the plurality of electrodes means for the off-axis beams are so structured and arranged as to cause a plurality of the field components to be asymmetric and effective to converge the off-axis beams. The strengths of the asymmetric field components vary in response to changes in the dynamic focus voltage. The asymmetric field components according to the invention have such polarity and strength, due to the structuring and arranging of the electrodes, and the application of the pattern of voltages, that a change in the levels of the dynamic focus voltage causes a change in the strength of each of the asymmetric field in a direction effective to additively deflect a common off-axis beam in a common angular direction so as to create a strong dependency of the convergence of the off-axis beams on variations in the focus voltage.
An electron gun according to the invention disclosed in copending application Ser. No. 832,568 comprises means including cathode means for developing an eletron beam. Main focus lens means provide for receiving the beam and forming a focused electron beam spot at the screen of the tube. The main focus lens means has a plurality of electrodes situated on a common axis. Means are provided for developing and applying to the electrodes potentials effective to form field components in the gaps between adjacent electrodes. The lens means is so structured and arranged as to cause at least one of the field components to be asymmetric and effective to significantly divert the beams from a straight-line path through a predetermined angle. Means for developing and applying a varying voltage to at least one of the electrodes causes the strength of the asymmetric field component, and thus the angle by which the beam is diverted, to vary.
Takenaka et al in U.S. Pat. No. 4,334,169 shows embodiments of an electron gun with a three-element main focus lens (G1, G2 and G3) and outer beam converging means at the field between the center electrode (G2) and the accelerating electrode (G3) of the main focus lens. The convergence means comprise offset apertures and apertures lying at an angle with respect to the gun axis to render the field between asymmetric. The G1 and G2 electrodes are electrically inked and receive the focusing voltage. An aperture electrode is located intermediate to G1 and G2 of the main focus lens and is electrically linked to the accelerating electrode of the prefocusing section. The object is stated to be the maintenance of the pre-established convergence of the outer beams, despite changes in the focusing voltage.
Other representative disclosures having electrode structures that influence beam convergence include:
U.S. Pat. No. 3,952,224 to Evans
U.S. Pat. No. 3,772,554 to Hughes
U.S. Pat. No. 4,473,775 to Hosokoshi et al
U.S. Pat. No. 4,513,222 to Chen
The performance of cathode ray tubes is also a function of the ability of the gun and associated systems to establish and maintain focus at all points on the screen. Conventional curved-screen, curved-mask tubes, because of the curvature of the screen, are able to attain tolerable focusing performance on all points on the screen with little or no dynamic focusing. However, tubes having a flat faceplate exacerbate the focusing problem particularly at the screen edges due to the lack of curvature of the screen. For high-performance flat-faced tubes, dynamic focusing of electron beams is very desirable.
Techniques for dynamically varying the focus of electron beams are well-known in the art. Dynamic focusing is used to cause a beam to be in focus at the sides of the picture imaging field as well as at the center of the field. The need for dynamic focusing arises from the aforedescribed accurate scanning of the beam with relation to the relatively planiform faceplate.
Dynamic focusing of a beam can be accomplished electronically by menns of a focus-control signal modulated at the scanning frequency, with the signal being applied to a suitable beam-focusing electrode. Dynamic focusing means is disclosed by Richard in U.S. Pat. No. 3,412,281. An A. C. control signal is employed which is proportional to the distortion due to defocusing inherent in tube faces, according to Richard. The A.C. control signal is converted into a D.C. control signal which may be added to the relatively high-level constant voltage of the tube focusing circuit. Another approach to dynamic focusing is disclosed by U.S. Pat. No. 2,801,363.
Three patents to Chen disclose astigmatism-forming electrode structures. In U.S. Pat. No. 4,234,814, a gun is described that has a screen grid with an aperture comprising a rectangular slot portion facing the control grid, and a circular portion facing away from the control grid. The slot portion of the apertures is said to create an astigmatic field that produces under-convergence of the beam in the vertical plane only to avoid and/or compensate for vertical flae distortion of the beam spot at off-center positions on the image screen. In U.S. Pat. No. 4,319,163 of Chen, a gun lower end is disclosed that includes a cathode, a control grid, a first screen grid electrode having a horizontally elongated rectangular aperture, and a second screen grid electrode having a circular aperture. ln operation, the second screen grid is energized with a DC bias voltage and the control grid and first screen grid is energized with a DC bias superposed with a substantially parabolically shaped dynamic signal synchronized with either or both the horizontal and vertical deflection signals. It is stated that the astigmatic optics of the beam forming means varies in strength in phase with the beam scan so as to provide optimum correction for flare distortion of the beam. In a third Chen Patent, U.S. Pat. No. 4,523,123, an inline gun includes a plurality of electrodes including a cathode, a control grid, a screen grid and a main focus lens. The screen grid has a given thickness with a plurality of transverse slots formed therein. The sots have a depth less than the thickness of the screen grid. An aperture is formed in each of the slots. The outer slots are asymmetric with respect to the apertures therein and are displaced transversely toward the center aperture. The transverse slots in the screen grid are said to compensate for the vertical flare distortion of the beam spot at off-center positions on the screen, and the asymmetric location of the outer slots is said to reduce the horizontal convergence sensitivity of the outer beams with respect to focus voltage change.
Other representative disclosures having astigmator electrode structures in the lower end include U.S. Pat. Nos. 4,242,613 to Brambring et al; 4,366,414 to Hatayama et al; and 4,629,933 to Bijma et al. Koshigoe in U.S. Pat. No. 4,641,058 discloses means for forming an asymetrical lens in both the prefocusing lens and in the main focus lens.
An in-line gun (the "DAF" gun) that is said to provide dynamic astigmatism and focus correction is described in a journal article by Suzuki et al. The focus electrode of a bipotential-type gun is split into a lower section adjacent to prefocusing lens, and an upper section adjacent to an accelerating anode. The beam-passing apertures in the opposed faces of the two sections are rectangular--the apertures in the ower section are vertically oriented, and those in the upper section are transverse to those in the lower section. The focus voltage is applied to the lower section, and a combination of the focus voltage and a dynamic voltage that is caused to vary with the excursion of the beams across the screen, is applied to the upper section. When the dynamic voltage is increased from the level of the focusing voltage, an electric quadrupole field is produced between the lower section and the upper section which is alleged to counter the astigmatizing field of the self-converging yoke. The amount of counter-astigmatizing is a function of the location of the beams on the screen--the farther from center screen, the greater the counter-astigmatizing effect. The lens formed between the upper section and the accelerating anode is an OLF--"overlapping field" lens. The use of OLF lens in this gun configuration is said to provide a larger apparent lens diameter with consequent lower magnification of the beam spots at center screen. However, the influence of the OLF lens is inherently astigmatizing, which distorts the beams at the center. Nor does the DAF gun have any provision for dynamic convergence other than the self-converging yoke. ("Progressive-Scanned 33-in. 110.degree. Flat-Square Color CRT." Suzuki et al. SID 87 Digest, pp 166-169.)